1. Field of the Invention
This invention concerns an apparatus for supplying offset printing presses with a damping solution.
2. Description of the Prior Art
Offset printing presses use a damping solution during the printing. Large printing presses will have complex equipment for supplying the damping solution. The use of a refrigeration unit to cool the damping solution to about 10 degrees C. offers considerable improvement in quality of the printing. Refrigerating the damping solution, however, adds to the complexity.
The addition of alcohol to the damping solution also provides many advantages. It lowers the surface tension of the solution, which insures that the printing plate is well damped. The alcohol gives a greater affinity of water to ink. Alcohol reduces the emulsification of the ink.
A disadvantage of using alcohol is that the evaporation rate is high, approximately eight times higher than water. This means that it can only be used with cooling units fitted with continual compensation devices. Because of the high price and high evaporation rate, efforts are being made to replace alcohol completely or in part by other agents, such an anionic detergents. Theses detergents lower the surface tension of the damping solution. These agents havve not completely repllaced alcohol, however, which has superior advantages as to quality.
Cooling systems currently available are capable of supplying much more damping solution than is necessary for small or medium format presses. These systems are complex and expensive. As a result, small and medium presses generally do not utilize cooling systems.
The damping trays of existing presses utilize a tray in which a ductor roller is mounted. The ductor roller is partially immersed in the damping solution and is driven. A take-up roller oscillates back and forth into contact with the ductor roller. The take-up roller has a sleeve of sponge like material which receives the damping solution from the ductor roller. The take-up roller oscillates over and distributes the damping solution onto a distributor roller.
While workable, the ductor roller rotating in the solution increases the rate of evaporation of the alcohol. Consequently, the damping solution, although concentrated at the beginning, arrives on the printing plate at a much lower concentration than the basic solution. Also, the solution which is not used by the damping of the printing plate will be transferred back to the damping tray via the ductor roller and the intermediate rollers. This also deconcentrates and contaminates the damping solution contained in the tray.